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MG MGA - Engine temperatures

Anyone who has had an interest in the various threads on engine temperature and the related issue of fuel delivery vapourisation should try and get to read the article by Paul Ireland in the February edition of MGCC's Safety Fast.

The article is titled 'Modern Petrol and Classic Engines' which relates Manchester University's tests on an XPAG engine using a heavily monitored test rig. Clearly I can't copy the article to the list but the summary is that older engines really do run hotter on modern fuels. This is fundamentally due to the fact that modern petrols include a wider range of hydrocarbons, not just the addition of ethanol, and they have a wider range of boiling points. It is the ones with lower boiling points that make the engines hotter, and with fuel line vapourisation there is the double issue that under bonnet temperatures are higher and that some components of the fuel boil at a lower temperature. BY the way the higher boiling point components are also an issue in different ways.

For me this explains my theory that we didn't have either of these problems in the early 70s when I was doing high mileage in my MGA.

For all it means there is a real issue and it is reasonable to implement solutions that go further than implemented by the factory.

There is a lot more interesting content in the article which I found very interesting as someone who started as a Chemical Engineer. There are several more articles due to come from this work in future.


Paul Dean

Paul, are they saying that modern fuels have a higher calorific value than older stuff? With 10% ethanol this makes no sense to me.
Art Pearse

Alcohol has about 40% less energy than gasoline. 10% alcohol in gasoline will have 4% less energy. To produce similar power, your engine needs to consume 4% more fuel (minimally). That should result in same heat production for same driving habits. Except the engine may produce 4% less power at full throttle. That alone should not make it run hotter.

But there are more differences. Could be differences in octane rating and/or burn rate which could affect required or desired ignition timing and compression ratio. If the burn happens later it could waste some fuel, waste some heat, run hotter. Change of ignition timing might help.

I would like to see the full report, but haven't been able to read it yet.
Barney Gaylord

Barney I think you might be calculating with much higher proof alcohol than what gets mixed in with the gasoline.

Full report says to advance timing. Nothing will bring back the lower power which equates to lower mileage from the ethanol.

Look to the State of California (not the gov't info) where if shows a much higher reduction in fuel mileage. Closet to 10 per cent. Same lower mileage when the MTBE was used in similar %'s. Next they want to use Toulene as the "oxygenate." Toulene, no kidding, won;t that be just dandy?

Oddly enough higher octane gasoline has fewer BTU's than lower octane gasoline, same volume. Real unadulterated gasoline, I mean. Not a whole lot of difference, but a difference none the less.
D Peltier

I have re read the article.

Firstly it is not just about alcohol as such and the point is there is a greater range of boiling points in the greater variety of hydrocarbons in modern fuel. This leads to components with lower and higher boiling points. A graph shows around 35C to 210C rather than the 75C to 150C of older fuels. On my re read I am actually wrong in part of my summary.

The article discusses in details what happens in a cylinder after a plug fires and how turbulence in the head is critical in spreading the burn. He then presents results that show individuals burn cycle vary quite significantly from cycle to cycle however good your tuning is. This 'cyclic variation' is the cause of under the bonnet over heating as on some cycles the burn will be too slow and as a result there will be an increase in the temperature of the hot gasses entering the exhaust (with some of the higher boiling point gasses still burning I presume this means). So in summary it is the higher boiling point components that cause higher under bonnet temperatures but the lower boiling point ones that vapourise giving us hot engine starting issues.

Of course there is more info to come in future articles and I hope I am correct in my summary of a highly technical article.

By the way there is a bit about alcohol re corrosion and the fact some fuel has contained alcohol since 1928 (Cleveland Discol). It is not particularly relevant to the above as it isn't near either end of the boiling point range.

Also from me not the article. I was always told high octane fuel had lower calorific values as more of it was non combustible additives.

Barney, I hope you can get to read the article.

Paul Dean

Not being a member of MGOC, I will likely never see the article. The problem with club newsletters is they are introverted and protected for members only, commonly no intention of ever releasing the information outside of the club. This is the primary reason for existence of my web site, information unencumbered and free to the world at large. That is, I do things that private clubs won't do. Feel free to visit
Barney Gaylord

I think that this is the link to the article.



R A Evans

Barney, I among many others, appreciate and benefit from your extensive work.

but may I correct a common misunderstanding ...

The MG Car Club is a members' club managed by the members. MGOC is a privately owned commercial business. Each contributes to and supports the hobby but in some different ways.

Ah, right to the source. I'm not a T-type guy, but already I like that web site. Very good information, and open to the public. I have made a couple of links to that article from my web site and from the Chicagoland MG Club web site.

In the first chart the top line represents the evaporative character of 5% alcohol in the fuel compared to non-alcohol fuel. It is a very significant difference. It would be nice if there was another curve for 10% alcohol fuel that is common in North America. I can imagine just push the top blue line "one step" higher.

I think I had a pretty good line on this before reading that article, but it is nice to see the data to back up the intuitive truth and real world experience. The problem of hot-start and parade-traffic fuel boiling has been obvious for years past.

The issue of higher temperature running at road speed is more vague. The article shows reason why the engine may run hotter due to slow-burn which results in late burn, but it is not well quantified. In particular, it doesn't seem to cover the possibility of advancing ignition timing to compensate for slower burning.

In short I'm not sure that modern fuel makes a huge difference in running temperature (given appropriate ignition timing), but alcohol sure does cause fuel boiling with conditions of low fuel flow an low vehicle speed (or short term parking). Vintage carburetors do not like alcohol in the fuel.

**** Sorry about my mis-speak in the prior post. Make that MGCC, not MGOC. ****
Barney Gaylord

This is another interesting article:

Nikolai Skliadnev

Toluene - is not an oxygenate! It is an aromatic hydrocarbon.
Barney is right that alcohol exhibits a vapour pressure way above what its bp might suggest when mixed in small proportions with hydrocarbons. This is because the molecules are quite dissimilar.
Art Pearse


I am glad Richard has found a version of the article that you can see. I had been wondering about contacting MGCC to see if I could share it to this limited group but no need now. It is very unusual for Safety Fast to have articles with such real technical content.

Thanks for all the work on your site, I use it all the time.

Paul Dean

Not yet got round to reading the articles and I admit to it not being a topic to which I can make any useful contribution. But that does not stop me asking a question. I presume this all revolves around current 85 grade fuel? I only ever put super unleaded 98 in my beast. Does the discussion read across to this grade and its contents?

Steve Gyles

The other referenced item by Nick covers octane issues.

Not withstanding that the T Type Register put the item onto an open website, had you asked the MGCC for permission to copy the article, I suspect they would have said no. The MGCC is on a drive to preserve the perceived value from club membership.

Colin Manley

Colin - No I didn't get as far as contacting them although I suspect you are right..

Barney/Steve/All - As I read the article the issue is the variation of cycles. Hence even if you are tuned correctly for a cycle that falls in the middle of the range, that is correct tune, some cycles will vary enough to cause the problem so you can't solve it completely by correct tune. The relevant point is not octane as such but the range of hydrocarbon boiling points in the mix. If the manucturer reduces this range in producing premium fuels I would say it should help, but they may not to do this. What the author says is that if you find fuel that suits you stick to it, and I guess this is your situation Steve. I think also the message this isn't all about alcohol as it's boiling point is nowhere near of the bottom the range shown in the chart. The presume the BP article is the source of this chart repeated in the article.

There is a lot of me saying 'I think' here so treat with caution!
Paul Dean

This thread was discussed between 08/02/2017 and 11/02/2017

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